In cardiac surgery, connections between the patient's vasculature and extracorporeal circulation are established and maintained by cannulae that serve as temporary conduits for blood flow. Conventional cannulae are semi-rigid polyvinyl chloride or polyurethane tubes positioned and secured by sutures in the patient's arterial and venous vessels. Since cannulae were first used in human cardiopulmonary bypass in the 1950's, the design of the most widely used arterial and venous cannula has not changed dramatically.
From early usage in the history of cardiac surgery, it has been widely appreciated that cannulae designs that maximize fluid flow are preferred. In general, surgeons purposely select the largest diameter cannula that can be atraumatically inserted into a blood vessel to increase perfusion to the body and drainage to the extracorporeal pump. In addition, utilizing a cannula of greater cross-sectional area reduces damage to blood elements as blood traverses along the cannula wall. Manufacturers have recently reduced the cannula wall thickness and added side-holes in the end of cannulae in attempts to increase flow at the cannula tip. However, less than optimal fluid flow characteristics persist if there are abrupt size discrepancies at any point along the path of blood flow.
Changes in cross-sectional area or shape such as a sudden expansion, contraction, or an angle in the stream of flow cause the fluid velocity to change direction or magnitude. This produces an increase in friction and turbulence as compared to fluid flow in a straight tube. Since cardiopulmonary bypass cannula function to form a connection between standard size extracorporeal tubing and blood vessels that vary in diameter and compliance, significant turbulence may result. The evolution of cannula to more tapered designs and wire reinforcement to prevent sharp angles reflect the attempts by industry to overcome these size and shape discrepancy problems.
At present, industry uses a standardized method to compare fluid flow and turbulence characteristics of various cannula designs by measuring “pressure drop.” The pressure drop is determined by measuring the change in pressure as compared to fluid flow perfusing through the cannula in a standardized test apparatus. In practice, each cannula design has a published pressure drop which is made available to the surgeon to allow comparison between cannula designs. Unfortunately, the industry's standard method of measuring the pressure drop provides limited information about fluid flow characteristics within the blood vessel, beyond the distal end of the cannula.
The importance of fluid flow characteristics beyond the cannula tip is exemplified by the so-called “sandblasting” effect described for aortic cannula perfusion during cardiopulmonary bypass. A jet of high velocity blood exiting the relatively small perfusion cannula (approximately 7-8 mm) into the adult ascending aorta (approximately 30 mm) has been identified as a source of dislodgement and mobilization of artherosclerotic atheroma to vital organs. This has been suggested as a potential mechanism of cerebral infarcts in patient's undergoing cardiac surgery. Various devices which include special tip cannula (Sarns softflow® cannula, 3M Healthcare, Ann Arbor, Mich., USA and Edwards DISPERSION® cannula, Research Medical Inc., Midvale, Utah, USA) or temporary filters (EMBOLEX®, Mountain View, Calif., USA) have been proposed to prevent this phenomenon or its associated morbidity.
The performance of cannulae are influenced by the biomechanical properties of the vessels they cannulate. Too rapid drainage of the venous system can cause intermittent collapse of the vein distal to the cannula. This can significantly reduce fluid flow and produces excessive cannula movement referred to as “chattering” as the vein repeatedly collapses and refills. This phenomenon may significantly impede venous drainage and disturb the surgical procedure by disrupting the surgical field with vibration and movement. In order to address this problem, specialized cannulae designed to eliminate collapse of the vein are commercially available, such as the swirl tip atrial caval venous cannula (Medtronic Corporation, Minneapolis, Minn., USA).
Venous cannulae generally tend to be a larger diameter and a longer length than arterial cannula. These design features are intended to compensate for the compliant nature of the veins and prevent vessel collapse. However, the increase in cannula length can significantly increase the resistance of the fluid path. In addition, minimally invasive cardiac surgery has led to a need for smaller diameter venous cannula for use with vacuum-assisted drainage which tends to exacerbate distal vein collapse.
Even with open surgical field exposure, judging the size of blood vessels can be difficult due to fluid pressure changes or vessel spasm following tissue dissection. Cannulation procedures performed with maximal surgical exposure can be less than optimal due to erroneous blood vessel sizing due to fluid pressure changes or vessel spasm during dissection. Minimally invasive cannulation increases the technical challenge of vessel sizing due to limited exposure. Overestimation of the size of the vessel may result in selection of too large of a cannula and excessive vessel trauma during cannulation. Underestimation of blood vessel size with insertion (of a smaller cannula may result in less than optimized fluid flow. Incorrect cannula size selection may delay the procedure and increase discarded, but unusable cannula.
Technological developments are increasingly challenging the limits of current cannula designs. Advancements in extracorporeal devices, such as ventricular assist devices (VAD) have increased the potential for size discrepancies between the patient's vasculature and the circuit. Minimally invasive techniques, such as extracorporeal membrane oxygenation (ECMO) and vacuum-assisted drainage, have increased the use of small peripheral blood vessels as cannulation sites.
Previous attempts have helped to solve narrow parts of the problems discussed above. However, there has not been a broad-spectrum advance in cannula design to address these problems in the wide range of applications in which cannulae are now commonly used. It would be desirable to construct a novel cannula to universally improve fluid dynamics and provide atraumatic insertion of cannula into blood vessels. It would be desirable that the cannula have a narrow insertion profile to ease insertion. Following placement, it would be desirable that a simple mechanism elicit the cannula to possess a wide internal diameter. It would be desirable for the device to exert an atraumatic force on the structures it cannulates to optimize the biomechanics of these structures for fluid flow. It would be desirable to have a cannula that would minimize or eliminate the reduction in fluid flow due to compliant vessel collapse. In certain applications, it would be desirable that the device completely occlude fluid flow through a blood vessel. In other applications, it would be desirable that the cannula selectively filter particles. It would also be desirable to have a single cannula that would replace the need for a number of different diameter cannulae to match the various sizes of the structures it cannulates.